Compressor Unit

ABSTRACT

The invention relates to a compressor unit, in particular for underwater operation, wherein the stator of the motor is connected to a separate cooling means. It is an object of the invention to provide a cooling means for the stator of the electric motor, which cooling means firstly affords excellent operational reliability and secondly does not require any exchange of materials with the surroundings during operation. To this end, there is provision for the cooling means to be configured in such a way that there is a natural circulation of a cooling medium in the cooling means of the stator under operating conditions. One decisive advantage of the compressor unit according to the invention lies in the fact that the separate cooling means of the stator can be adapted precisely to the operating conditions of the latter and, in particular, firstly the high power loss and secondly the sensitivity of this component can be taken into consideration.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International Application No. PCT/EP2007/051539, filed Feb. 19, 2007 and claims the benefit thereof. The International Application claims the benefits of European application No. 06006066.2 filed Mar. 24, 2006, both of the applications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

The invention relates to a compressor unit for compression of a pumping medium, in particular for underwater operation, comprising a compressor and an electric motor which comprises a stator and a rotor.

BACKGROUND OF THE INVENTION

Recent developments in the field of compressor design have also been concentrated on undersea arrangements of large compressors which are intended to be used for the pumping of natural gases.

Because of the particular operating conditions, in particular because of the greatly restricted accessibility both for maintenance purposes and by means of supply lines, the specialists are confronted with major requirements. The relevant environmental regulations forbid any exchange of substances between the equipment to be installed and the surrounding sea water. Furthermore, sea water is an aggressive medium and extreme pressure and temperature conditions can be found at the various depths in the sea. A further requirement is that the equipment should on the one hand have an extremely long life and on the other hand must be designed to be virtually free of maintenance. An additional exacerbating factor is not-inconsiderable contamination of the medium to be pumped which in some cases is chemically aggressive.

A compressor unit of the abovementioned type is already known from international patent application WO 02/099286 A1. The compressor unit described there provides, for cooling purposes, that a portion is tapped off from the pumping medium, generally natural gas, in the area of an overflow from the radial stages of the compressor and is used to pass around the components to be cooled, in such a way that the heat losses, which are in the order of magnitude of 100-200 kW, are dissipated with the cold medium to be pumped.

A compressor unit of the type mentioned initially is already known from EP 1 069 313 A2, whose electric motor has a separate cooling arrangement which is operated with nitrogen as a cooling medium, which is pumped in a gaseous form in the circuit by means of an additionally driven apparatus.

A fluid energy machine in which a cooling medium is pumped in a forced circulation by means of a pump is already known from DE 196 23 553 A1.

This concept for cooling the compressor unit is particularly advantageous since the pumping medium which must be transported in any case is used to dissipate the heat losses and there is no need for any additional media exchange between the compressor unit and further components of the environment. However, this procedure results in particular difficulties owing to the aggressive chemical characteristics of the media to be pumped. While it is sufficient for flow to be passed around the rotor for cooling, further cooling measures are necessary to dissipate the heat losses for the stator.

A further risk is the generally porous insulation of the stator which absorbs part of the pumping medium in the area where the pumping medium flows around it, while the pumping medium is in contact with the stator as a cooling medium, in such a way that, in the event of a sudden pressure drop, for example in the case of an interruption in operation, this leads to explosive expansion of the absorbed medium in the pores of the insulation, which is in consequence destroyed.

The described disadvantages with their high risks for the availability of a compressor unit are unacceptable in particular for underwater operation, for example for the pumping of natural gas.

SUMMARY OF INVENTION

The invention is therefore based on the object of providing a cooling arrangement for the stator of an electric motor of a motor-driven compressor unit, in particular for undersea operation, which on the one hand offers excellent operational reliability and on the other hand does not require any substances to be exchanged with the environment during operation.

A compressor unit is proposed in order to solve this problem.

One major advantage of the compressor unit according to the invention is that the separate cooling of the stator can be matched precisely to its operating conditions and, in particular, it is possible to take account on the one hand of the high power losses and on the other hand of the sensitivity of this component.

In particular, the sensitivity of the-stator to contamination is taken into account if the cooling arrangement has a closed circuit in which a cooling medium circulates. One expedient development of the invention provides for the stator to be provided with cooling channels and for the pumping medium to flow through these channels for cooling. When using the pumping medium for cooling purposes, this embodiment involves the risk that the contamination of the pumping medium during the course of operation adversely affects the flow through the channels, and may also block them. The stator cooling arrangement is operationally reliable because of the separate circuit according to the invention.

It is worthwhile for the cooling arrangement to have a condenser which is connected to the cooled stator by means of a feed line and a return line, with the cooling arrangement being designed such that the cooling medium circulates between the condenser, the return line, the stator and the feed line. The circulation can be driven particularly advantageously by means of natural convection, thus resulting in a natural circulation of the cooling medium between the abovementioned components. This makes it possible to save additional pumping precautions for the cooling medium, thus reducing the complexity of the compressor unit and in this way contributing to high availability. In order to ensure a particularly wide thermal range of operation, the circulation in the cooling arrangement can also be driven by means of a pump in such a way that this always results in a forced circulation.

The cooling medium is particularly advantageously in such a form that a phase change of at least a part of the cooling medium takes place in the circulation in the cooling arrangement. This results in the cooling performance being particularly high. The pressure of the filling of the cooling medium in the closed circuit can be set such that, in operating conditions, at least a portion of the cooling medium changes to the gas phase during the heat absorption in the stator, and this portion changes back to the liquid phase during the heat emission in the condenser.

Ammonia, carbon dioxide and hydrocarbons are highly suitable for use as a cooling medium. The hydrocarbons may be both halogenated and non-halogenated, in which case the non-halogenated hydrocarbons are advantageous over the halogenated hydrocarbons, in the same way as ammonia and carbon dioxide, for environmental compatibility reasons.

For the purposes of the invention, separate cooling for the stator of the electric motor of the compressor unit, on the one hand, and a cooling system for the further elements of the compressor unit, on the other hand, are particularly expedient. The separation of the cooling from the cooling system is appropriate for the particular requirements for the heat dissipation from the stator of a compressor unit of this generic type.

The cooling system which, inter alia, cools the compressor and the rotor of the motor as well particularly advantageously provides the pumping medium as the cooling medium, as a result of which the heat losses are dissipated with the pumping medium

to be compressed. This is particularly advantageous for undersea pumping of natural gas, since this is generally relatively cold.

It is particularly worthwhile for the pumping medium to flow around the rotor in the open circuit.

The pumping medium is frequently heavily contaminated and can adversely affect the operation of sensitive components as it flows around them. It is therefore worthwhile designing the bearings of the rotor, specifically the axial bearings and the radial bearings, in an encapsulated form, such that no substances are exchanged between the environment and these components. This means that magnetic bearings must be used. This also applies to the rotor and the stator, which can be protected in a similar manner by means of encapsulation against the aggressive pumping media.

In order to ensure that there is no need to design the cooling arrangement of the stator in a costly manner for the maximum pressure difference—resulting on the one hand from the externally applied pressure of the cooling pumping medium which is used in the area around the stator in order to cool the other components of the compressor unit by means of the cooling system, and on the other hand the internal pressure of the cooling arrangement—it is worthwhile providing different operating pressures for the cooling arrangement for different pressures from the pumping medium, and to change the cooling medium for cooling as a function of the operating pressure. The specific application in which natural gas is being pumped, this means that the pressure of the pumping medium at the inlet may vary between 40 bar and 140 bar depending on the amount and yield of the deposit, and therefore that the cooling arrangement should be designed for a differential pressure of up to 200 bar if the cooling arrangement is operated just with the same cooling medium over the entire period during which the natural gas is being pumped. Advantageously, it is possible to design for only a reduced pressure difference if the cooling medium is changed as a function of the pressure of the pumping medium, for example in the sequence propane, butane, freon. The change in the cooling medium can advantageously be synchronized with other maintenance tasks.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail in the following text using one specific exemplary embodiment and with reference to drawings, in which:

FIG. 1 shows a schematic illustration of a longitudinal section through a compressor unit according to the invention, and

FIG. 2 shows a schematic illustration of the motor of the compressor unit with the separate cooling arrangement, as a thermosiphon.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 shows, schematically, a section along a compressor unit 1 according to the invention which has, as major components, a motor 2 and a compressor 3 in a gas-tight housing 4. The housing 4 accommodates the motor 2 and the compressor 3. The housing 4 is provided with an inlet 6 and an outlet 7 in the area of the junction between the motor 2 and the compressor 3, with the fluid to be compressed being sucked in through the inlet 6 by means of a suction connecting stub 8, and with the compressed fluid flowing out through the outlet 7.

The compressor unit 1 is arranged vertically during operation, with a motor rotor 15 of the motor 2 above a compressor rotor 9 of the compressor 3 being combined to form a common shaft 19 which rotates about a common vertical rotation axis 60.

The motor rotor 15 is borne in a first radial bearing 21 at the upper end of the motor rotor 15.

The compressor rotor 9 is borne by means of a second radial bearing 22 in the lower position.

An axial bearing 25 is provided at the upper end of the common shaft 19, that is to say at the upper end of the motor rotor 15. The radial bearings and the axial bearing operate electromagnetically and are each encapsulated. In this case, the radial bearings extend around the respective bearing point of the shaft 19 in the circumferential direction and in this case are circumferential through 360° and are undivided.

The compressor 3 is in the form of a centrifugal compressor and has three compressor stages 1 which are each connected by means of an overflow 33. The pressure differences which result across the compressor stages 11 ensure that there is a thrust on the compressor rotor 9 which is transmitted on the motor rotor 15 and is directed against the force of gravity from the entire resultant rotor comprising the compressor rotor 9 and the motor rotor 15, thus resulting in a very high degree of thrust matching during rated operation. This allows the axial bearing 25 to be designed to be comparatively smaller than if the rotation axis 60 were to be arranged horizontally.

The electromagnetic bearings 21, 22, 25 are cooled to the operating temperature by means of a cooling system 31, with the cooling system 31 providing a tap 32 in an overflow of the compressor 3. A portion of the pumping medium, which is preferably natural gas, is passed from the tap 32 by means of pipelines through a fitter 35, and is then passed through two separate pipelines to the respective outer bearing points (first radial bearing 21 and fourth radial bearing 24 as well as the axial bearing 25). This cooling by means of the cold pumping medium saves additional supply lines.

The motor rotor 15 is surrounded by a stator 16 which has encapsulation 39 such that the aggressive pumping medium does not damage the windings of the stator 16. The encapsulation is in this case preferably designed such that it can contribute to the full operating pressure. This is also because a separate cooling arrangement 40 is provided for the stator, in which cooling arrangement 40 a dedicated cooling medium 56 circulates. A pump 42 in this case ensures circulation via a heat exchanger 43, assisting the natural circulation.

At least the encapsulation 39 is designed such that the section which extends between the stator 16 and the motor rotor 15, while having a thin wall thickness, is nevertheless able to withstand the design pressure when the stator cooling arrangement 40 is completely filled by means of the cooling medium 56. This makes it possible to avoid relatively high eddy current losses in this area, thus improving the efficiency of the overall arrangement.

Depending on the pressure of the pumping medium, the pressure of the filling or the cooling medium 56 in the stator cooling arrangement 40 can be matched such that the encapsulation is always operated in the design range of the pressure difference. The cooling medium 56 can be correspondingly changed during maintenance tasks, for example in the sequence from propane to butane to freon, in the sequence of falling pressure.

The compressor rotor 9 expediently has a compressor shaft 10 on which the individual compressor stages 11 are mounted. This can preferably be done by means of a thermal shrink fit. An interlock, for example by means of polygons, is likewise possible. Another embodiment provides for different compressor stages 11 to be welded to one another, thus resulting in an integral compressor rotor 9.

FIG. 2 shows the motor rotor 15, the stator 16 and the cooling arrangement 40. The cooling arrangement 40 has a cooling circuit 50 which extends through cooling channels 51, collecting areas 52 arranged on both sides of the cooling channels 51, into lines which connect these collecting areas, specifically a feed line 53 and a return line 54, as well as a condenser 55 arranged between the feed line 53 and the return line 54. The cooling medium 56, for example a hydrocarbon, starts to flow in cooling channels 51 of the stator 16, flows through the feed line 53 into the condenser 55 where the cooling medium 56 is condensed, and then flows as a liquid through the return line 54 into a collecting area 52 which is located at the return end of the cooling channels 51. The circuit is closed and the pressure and the amount with which it is filled are chosen such that a phase change takes place between the feed and return. The temperature difference between the feed and the return is preferably 10 K. The condenser is located geodetically at the highest point (height difference ΔH), thus allowing a natural circulation. A pump 42 can be arranged in the return to assist this. The stator is encapsulated, and cooling by means of the pumping medium 80 which flows around the rotor 15 takes place in a gap between the rotor 15 and the stator 16. 

1.-14. (canceled)
 15. An underwater compressor unit for compression of a pumping medium, comprising: a compressor and an electric motor having a stator and a rotor; and a cooling arrangement separate from and connected to the motor stator where the cooling arrangement cools the motor stator and is constructed and arranged such that a cooling medium naturally circulates in the cooling arrangement of the stator during operation, wherein the cooling arrangement has a condenser connected to the cooled stator via at least one feed line and at least one return line where the cooling medium circulates between the condenser, the return line, the stator and the feed line.
 16. The compressor unit as claimed in claim 15, wherein the cooling medium circulates within a closed circuit of the cooling arrangement.
 17. The compressor unit as claimed in claim 16, further comprising a pump arranged in the return line that pumps the cooling medium to produce a forced circulation of the cooling medium.
 18. The compressor unit as claimed in claim 16, wherein the cooling medium, in the closed circuit in the operating conditions and at the operating pressure, at least partially, changes to the gas phase during the heat absorption in the stator, and with the cooling medium changing to the liquid phase during the heat emission in the condenser.
 19. The compressor unit as claimed in claim 18, wherein the cooling medium is a hydrocarbon.
 20. The compressor unit as claimed in claim 15, wherein the compressor, the bearings of the compressor unit and the rotor of the motor are connected to a further cooling system and are cooled via the further cooling system.
 21. The compressor unit as claimed in claim 20, wherein the a cooling medium is a pumping medium.
 22. The compressor unit as claimed in claim 21, wherein the pumping medium is natural gas.
 23. The compressor unit as claimed in claim 22, wherein the cooling medium flows around the motor rotor.
 24. The compressor unit as claimed in claim 23, wherein walls of the cooling arrangement are adjacent to the cooling pumping medium and are designed for a maximum differential pressure between the pressure of the pumping medium and the pressure of the cooling medium in all operating states.
 25. The compressor unit as claimed in claim 23, wherein walls of the cooling arrangement are adjacent to the cooling pumping medium and are not designed for a maximum differential pressure between the pressure of the pumping medium and the pressure of the cooling medium in all operating states, and the pressure of the cooling medium is increased or decreased as a function of the pressure of the pumping medium.
 26. A method for operation of an underwater compressor unit for compression of a pumping medium, comprising: providing a compressor and an electric motor having a stator and a rotor; providing a cooling arrangement separate from and connected to the motor stator where the cooling arrangement cools the motor stator and is constructed and arranged such that a cooling medium naturally circulates in the cooling arrangement of the stator during operation, wherein the cooling arrangement has a condenser connected to the cooled stator via at least one feed line and at least one return line where the cooling medium circulates between the condenser, the return line, the stator and the feed line, and when the pressure of the pumping medium at the inlet varies, the cooling medium in the cooling arrangement is changed, and when the pressure of the pumping medium at the inlet is reduced, the operating pressure of the cooling medium in the cooling arrangement is also reduced.
 27. The method as claimed in claim 26, wherein, when the operating pressure of the cooling medium changes, the cooling medium is replaced with a different cooling medium. 